The geosynclinal development of the Rheinische Schiefergebirge (Rhenohercynian Zone of the Variscides; Germany)

Abstract

The Palaeogeographical distribution and evolution of facies in the Rheinische Schiefergebirge reflect the geosynclinal development of the Mid-European part of the Rhenohercynian zone of the Variscan geosyncline. The geosynclinal development of the Rheinische Schiefergebirge was strongly influenced by the Mid-Cerman Crystalline Rise in the south and by the Old Red Continent in the north. From late Precambrian up to Early Ordovician, the Mid-German Crystalline Rise has been a zone of rapid accumulation of sediments and volcanics, thus indicating an early tensional phase and the beginning of fracturing and mobilization of the European crust. With that the Variscan geosyncline proves to belong to a Proterozoic-Palaeozoic, resp. Caledonian-Variscan megacycle. The tension of the crust did not lead to the creation of an ocean in the described area. The Mid-German Crystalline Rise delivered debris into the southern part of the Rheinische Schiefergebirge during the Early Devonian. But the main part of this episialic sea was characterized by a large deltaic spread off the Old Red Continent. The delivered material accumulated in rapidly subsiding shelf troughs which followed asymmetrical graben-like structures and which retreated episodically northward. Tension at this time is also proved by an important volcanic activity. Outside of the shelf areas relatively thin pelagic sediments were deposited in a relatively stable basin of no great depth. The Givetian transgression caused a maximum of reef development. After the suppression of reefs by another global event in the late Frasnian, the further development led to an equalization of relief and facies. The pre-flysch phase in the Dinantian is characterized by another tensional episode with basic volcanism and by regionally limited deposition of limestone turbidites. The first occurrence of flysch sediments, delivered from the Mid-German Crystalline Rise, is already low in the Upper Devonian. A prominent acceleration of flysch accumulation and of northward shifting of the trough began in the late Dinantian. Already during the Namurian the flysch trough gradually changed into a molasse trough with paralic conditions